Python __mul__() Magic Method

Syntax

object.__mul__(self, other)

The Python __mul__() method is called to implement the arithmetic multiplication operation *. For example to evaluate the expression x * y, Python attempts to call x.__mul__(y).

We call this a “Dunder Method” for Double Underscore Method” (also called “magic method”). To get a list of all dunder methods with explanation, check out our dunder cheat sheet article on this blog.

Example

In the following example, you create a custom class Data and overwrite the __mul__() method so that creates a new Data object with the value being the product of the values of the two operands a and b of type Data.

class Data:

    def __init__(self, value):
        self.value = value
        
    def __mul__(self, other):
        return Data(self.value * other.value)


a = Data(21)
b = Data(2)
c = a * b

print(c.value)
# 42

You have defined the dunder method so that the resulting product of two Data objects is a Data object itself:

print(type(c))
# <class '__main__.Data'>

If you hadn’t defined the __mul__() method, Python would’ve raised a TypeError.

How to Resolve TypeError: unsupported operand type(s) for *

Consider the following code snippet where you try to multiply two custom objects without defining the dunder method __mul__():

class Data:

    def __init__(self, value):
        self.value = value


a = Data(21)
b = Data(2)
c = a * b

print(c.value)

Running this leads to the following error message on my computer:

Traceback (most recent call last):
  File "C:\Users\xcent\Desktop\code.py", line 9, in <module>
    c = a * b
TypeError: unsupported operand type(s) for *: 'Data' and 'Data'

The reason for this error is that the __mul__() dunder method has never been defined—and it is not defined for a custom object by default. So, to resolve the TypeError: unsupported operand type(s) for *, you need to provide the __mul__(self, other) method in your class definition as shown previously:

class Data:

    def __init__(self, value):
        self.value = value
        
    def __mul__(self, other):
        return Data(self.value * other.value)

Related Video

Python __mul__ vs __rmul__

Say, you want to multiply two objects x and y.

print(x * y)

Python first tries to call the left object’s __mul__() method x.__mul__(y). But this may fail for two reasons:

  1. The method x.__mul__() is not implemented in the first place, or
  2. The method x.__mul__() is implemented but returns a NotImplemented value indicating that the data types are incompatible.

If this fails, Python tries to fix it by calling the y.__rmul__() for reverse multiplication on the right operator y. If this method is implemented, Python knows that it doesn’t run into a potential problem of a non-commutative operation. If it would just execute y.__mul__(x) instead of x.__mul__(y), it could cause an error if the multiplication is non-commutative. That’s why y.__rmul__(x) is needed which indicates that multiplication is possible after all.

So, the difference between x.__mul__(y) and x.__rmul__(y) is that the former calculates x * y whereas the latter calculates y * x — both calling the respective multiplication method defined on object x.

References:

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